1. Technical Field
The present invention relates to novel olefinic block copolymers suitable for use as materials for the production of films having excellent heat sealability, transparency and blocking resistance.
Films made of polypropylene have been widely utilized as packaging material for foods, textiles and the like because they exhibit excellent mechanical, optical, and thermal properties. In the case where the polypropylene film is formed into a container such as a bag or it is sealed after packaging, the heat sealability of the film has a great influence on the packaging speed, the occurrence of imperfectly sealed products, a change in the quality of a packaged product and the like. Therefore, whether the polypropylene film is used as a single film or whether it is used as a composite film having a layer capable of being easily heat sealed on the surface thereof, there is a strong demand for improvement of the heat sealability of the film.
Furthermore, the heat sealability of a film is closely related to the melting/crystallizing property of the resin of which the film is made. It is known that the lower the melting or crystallizing temperature, the lower is the heat seal temperature at which the desired seal strength is obtained.
On the other hand, among the properties required for a packaging film, for example, the blocking property tends to become inferior as the heat sealability is improved. The main point of technical improvement is that the heat sealability of the film be improved without deterioration of the blocking resistance thereof.
Moreover, a packaging film is required to be amply transparent. Even if the heat sealability of the film can be improved at the expense of the transparency thereof, the resultant film will be of a low commercial value.
2. Prior Art
Heretofore, a number of crystalline random copolymers of propylene and ethylene have been proposed as resins for polypropylene films having good heat-sealability.
For example, U.S. Pat. No. 3,671,383 and Japanese Patent Publication No. 49-14343 disclose examples of a copolymer of propylene and ethylene which contain at least 75% of propylene or a crystalline random coplymer of propylene and ethylene which contains 1 to 6% of ethylene. However, with these copolymers, the heat seal temperature applied to obtain a heat seal strength necessary for practical purposes is not sufficiently low. In addition, as the ethylene content is increased, the heat sealability is certainly improved to a slight degree, but the blocking resistance rapidly deteriorates. Therefore, such a copolymer is useless for practical purposes. Moreover, the transparency begins to become inferior.
The quantities set forth above as well as others herein expressed in percent (%) and other quantities herein expressed in "part(s)" are by weight unless otherwise indicated.
U.S. Pat. No. 4,252,851 discloses that a copolymer of propylene and a C.sub.4-10 .alpha.-olefin, preferably, butene-1, which has a propylene content of 80 to 95% based on the weight of the copolymer is preferable for an easily heat-sealable layer. Supplementary tests we carried out also revealed that a copolymer of propylene and butene-1 exhibited an improved heat sealability only when the butene-1 content is maintained at a high level of 10% or more. Even in this case, the heat seal temperature required to obtain a seal strength of 500 g per 20 mm width, as determined by the method of the present invention described hereinafter, is of the order of from 130.degree. to 135.degree. C. The transparency seemed to be equal to or slightly poorer than that of the propylene-ethylene copolymer.
Japanese Patent Application Laid-open No. 52-11281 states that a crystalline random copolymer comprising 86 to 98.9% of propylene, 1 to 10% of a straight-chain .alpha.-olefin other than propylene and 0.1 to 4.0% of ethylene is suitable for use as an easily heat-sealable layer. However, in accordance with the description of the examples and our supplementary tests, it seems that the invention described in this publication provides only a film having a heat seal temperature of 130.degree. C. It was confirmed by our supplementary tests that if the ethylene and .alpha.-olefin contents are increased within the limits described in the publication in order to reduce the minimum heat sealable temperature, the nature of the copolymer particles deteriorates remarkably, and at the same time, the product film exhibits a remarkably high blocking property.